Nanocarbon Shows Promise for Fluorescent Bioimaging

A team from Nagoya University and Boston College has developed a flexible, water-soluble warped nanographene molecule that is biocompatible. The molecule was also found to induce cell death when exposed to light. The new molecule could expand the biological applications for nanocarbons, including cancer cell imaging and treatment.

Hydrophilic chains on the periphery impart high water solubility. Courtesy of ITbM, Nagoya University.A warped nanographene molecule was synthesized by the team in 2013 that is soluble in most common organic solvents and that exhibits green fluorescence when irradiated with UV or blue light. The team added water-soluble functional groups to the molecule to enable its use for biological applications.

When the team examined the fluorescent properties of water-soluble warped nanographene, it found that under UV light, the molecule fluoresced yellow when dissolved in water, and fluoresced green when mixed in the common organic solvent dichloromethane. The new nanographene molecule showed high photostability. Its properties did not change when exposed to light — rather, the color of fluorescence changed according to the polarity of the solvents in which the molecule was dissolved.

Next, the team tested to see whether the new molecule could stain live cells for fluorescent cell imaging. Researchers treated cells with a water-soluble warped nanographene solution. Microscopic observations showed that the cells took up the molecule over the course of a few hours, and that the molecule accumulated in the lysosomes.

Cell viability did not change significantly over time, demonstrating that water-soluble warped nanographene has low cytotoxicity and could be used as a fluorescent stain for HeLa cells. However, when the treated HeLa cells were irradiated with a blue laser, they exhibited cell death after 30 minutes. Untreated HeLa cells did not.

Common nanographenes are stacked in solvents, whereas warped nanographenes are dispersed in solvents. Courtesy of ITbM, Nagoya University.
“Although our new warped nanographene has low toxicity to HeLa cells, we were surprised to find that cell death was observed upon irradiating light to the cells stained with the new nanographene,” said professor Kenichiro Itami.

The specific mechanism of how this cell death occurs is not yet clear to the researchers, but the group speculates that a toxic singlet oxygen molecule generated during irradiation is responsible for cell death.

The warped nanographene undergoes C-H borylation to add boron moieties, followed by the Suzuki coupling to add water-soluble chains to the molecule (TEG = tetra(ethylene) glycol). Photo images of an ultraviolet irradiated quartz cell containing the water-soluble warped nanographene dissolved in various solvents (dichloromethane: green and water: yellow fluorescence). Courtesy of ITbM, Nagoya University.
The researchers believe that their method to functionalize and tune warped nanographenes could lead to biocompatible molecules that absorb different wavelengths of irradiation. Such molecules could be safely used to treat cancer cells in deep tissues. Further investigation is required to determine how nanocarbons could be used for a range of biological applications, such as photodynamic therapy for cancer treatments.

“We have succeeded in synthesizing a water-soluble warped nanographene showing fluorescence, good photostability and low cytotoxicity, which makes it promising for bioimaging,” said Itami. “We hope that our molecules can be developed further for a wide range of biological applications through further interdisciplinary collaborations.”

The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.